A pressure sensor of the type mentioned above is known from German Patent Publication (DE-OS) 2,263,901 (Andersson). The known pressure sensor has a pressure-sensitive membrane with a thickened inner area and a thickened outer area. Reaching between the inner area and the outer area are ridge-shaped upright measuring beams of constant cross-section. These measuring beams are rigidly secured to the membrane and loaded by shear stress. The measuring beams are provided with strain gages which measure the deformations. Although such pressure sensors exhibit greater sensitivity than pressure sensors with membranes without measuring beams, the attainable linearity is limited in pressure sensors as disclosed by Andersson. German Patent Publication (DE-OS) 2,263,901 contains explanations indicating that measuring transducers that function with measuring beams subject to bending stress are inferior to transducers in which the measuring beams are subject to shear stress. U.S. Pat. No. 3,712,123 (Laimins) discloses a compound plate strain gage transducer having a low profile and capable of measuring tension or compression loads. In the Laimins transducer a diaphragm (16) is formed in the compound plate, e.g. by four closed-end holes (12, 13, 14, 15) spaced by 90.degree. from one another to form ribs (23, 24, 25, 26) as sensor beams carrying strain gages (19, 20, 21, 22). The ribs or beams have sloping surfaces (10, 11) to provide a rib thickness which changes in the radial direction to have a minimum thickness about halfway between a rim (7) and a hub (8). The closed-end holes serve two purposes. First, the holes determine with their diameter the size of the ribs and thus the mechanical strength of the transducer. Second, the holes determine with their depth the thickness of the sensitive portion of the membrane and thus the sensitivity of the transducer.
In the Laimins transducer the strain gages are applied to said sloping rib surfaces. This is undesirable because securing the strain gages in exact positions on sloping surfaces is difficult. The strain gages are secured by an adhesive and slipping of any individual strain gage as it is being applied to the sloping beam surface, is hard to avoid until the adhesive has cured. Thus, Laimins leaves room for improvement, especially with regard to avoiding special hold-down tools for the mounting of the strain gages in precise positions. U.S. Pat. Nos. 4,376,929 (Myhre) and 4,133,100 (Myhre) disclose a modified diaphragm for a load cell of the Laimins type, in which the plurality of dead-end holes has been replaced either by a single hole with two diametrically opposite lobes, or with intricate depressions, or with a large number of holes or depressions for controlling the stress distribution, the sensitivity and other transducer qualities including a mechanical amplification in the output signal generation. U.S. Pat. No. 4,133,100 relates especially to the manufacturing of such diaphragms by means of chemical etching. The precise placing of the strain gages is difficult.
U.S. Pat. No. 3,035,240 (Starr) discloses a diaphragm compression sensing load cell with a diaphragm having one or more sensor beams integrally formed with the diaphragm held in a spring element which in turn is mounted between two housing plates. The main purpose of Starr is to protect the strain gages from the working fluid and external effects. For this purpose the strain gages are to be mounted in end bores of the sensing beam. This type of mounting is also hard to accomplish with precision in practice.